Experimental and numerical studies on the microbial restoration of natural gas deposits in depleted oil reservoirs storing CO₂

We investigated the microbial conversion of CO₂ into CH ₄ in depleted oil reservoirs by the interaction between indigenous oil-degrading hydrogen-producing bacteria and indigenous hydrogenotrophic methanogens that have been found in oil reservoirs universally. In this study, we investigated the influence of crude oil, yeast extract, bicarbonate and CO₂ on the growth and gas production of those bacteria through the incubation experiments of isolated strains under reservoir conditions. The yeast extract was estimated to be the most influential factor on the growth and gas production of oil-degrading hydrogen-producing bacteria. Methanogen was unaffected by the crude oil, the yeast extract and bicarbonate, however, both CO₂ and H₂ are assumed to be the influential factors on it because they are the energy sources of methanogen. In addition, we also investigated their growth kinetics that were needed to construct a numerical simulator of the microbial conversion of CO₂ into CH₄ in depleted oil reservoirs. The specific growth rate of oil-degrading hydrogen-producing bacteria was increased as the yeast extract concentration increased while that of the methanogen was constant regardless of the yeast extract concentration. These results indicate that the growth of methanogens is unaffected by the yeast extract that is injected into reservoirs to stimulate the growth of ODHPB. The growth yield of HYH-8 and HYH-10 was 5.5×10 ¹⁰ cells/g-(yeast extract) and 3.5×10¹¹ cells/g-(yeast extract) respectively.